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Identification of a heavy Higgs

 The identification of a Standard Model Higgs particle with a mass above 650 GeV will not be possible in the golden decay channel H $\rightarrow$ ZZ $\rightarrow$ l +l -l +l -: the cross section is too low to observe the four lepton decay mode with its low branching ratio of. As seen in fig. 2.6 the vector boson decays are dominant for Higgs masses above 300 GeV, hence the only alternatives to the four lepton decay are decay channels with a higher branching ratio in the vector boson decays.

The analysis presented here will focus on a 1 TeV Higgs which, as described in detail in section 2.5, is close to or above the upper theoretical limit for a standard model Higgs particle. The branching ratios for a 1 TeV Standard Model Higgs particle can be found in table 8.1

Table 8.1: The dominant branching ratios for a 1 TeV standard model Higgs particle.
Decay Branching ratio
H $\rightarrow$ W +W - 0.624
H $\rightarrow$ W +W - $\rightarrow$ 4j 0.296
H $\rightarrow$ W +W - $\rightarrow$ l$\nu$jj 0.144
H $\rightarrow$ W +W - $\rightarrow$ l +$\nu$l -$\bar{\nu}$ 0.029
H $\rightarrow$ ZZ 0.309
H $\rightarrow$ ZZ $\rightarrow$ 4j 0.151
H $\rightarrow$ ZZ $\rightarrow$ l +l -jj 0.029
H $\rightarrow$ ZZ $\rightarrow$ l +l -$\nu$$\bar{\nu}$ 0.008
H $\rightarrow$ ZZ $\rightarrow$ l +l -l +l - 0.001
H $\rightarrow$ t$\bar{t}$ 0.067

The simulation of the physics events was done with PYTHIA 5.7 [74] using the CTEQ(2L) structure functions. The fast analysis program ATLFAST [75] was used for the analysis. ATLFAST is a parameterization of the ATLAS detector allowing studies of physics processes in ATLAS without a the full and time consuming detector simulation. From the output of a physics event generator ATLFAST deposits the energy of the stable particles on a grid in ($\eta$,$\varphi$) with a resolution corresponding to the basic resolution of the ATLAS detector. The deflection in the solenoidal field is taken into account for charged particles. With a given cone size the program finds clusters of cells with energy above a threshold inside the cone and label them as jets if the summed energy inside the cone pass an adjustable threshold. In the case of overlapping cones the energy of the shared cells are shared according to the jet energy. Isolated electrons and muons are identified and the energies of jets, electrons and muons smeared according to parameterizations calibrated with full simulations of the ATLAS detector. The ATLFAST program was modified to allow the reconstruction of jets with different cone sizes simultaneously and to include the calculation of jet invariant masses.

Comparisons between simulations with ATLFAST and full detector simulations can be found in [75,76]. In general the agreement is good but special care should be taken in situations where pile-up can have a strong influence on the results.

next up previous contents
Next: Reducible backgrounds Up: thesis.html Previous: Summary of the #math596#
Ulrik Egede